The imaginary line on the surface of the Earth, running between the north and south poles, that demarcates one calendar day from the next. It passes through the middle of the Pacific Ocean, roughly along the 180° longitude, opposite the Prime Meridian, but it is drawn with diversions to pass around some territories and island groups.
So a traveler crossing the IDL eastbound subtracts one day, or 24 hours, so that the the calendar date to the west of the line is repeated. Crossing the IDL westbound results in 24 hours being added, advancing the calendar date by one day. The line is necessary in order to have a fixed, albeit arbitrary, boundary on the globe where the calendar date advances in the westbound direction.
That's what you think is it?
No. You are wrong. Today I have learned that the International Date Line is:
"... a snare of Satan brought in by his own agents to confuse minds."
And who beleives this?
Why it's our old friend, Ellen G. Wright, one of the founders of Seventh Day Adventism.
I'd say you can't make this crap up, but she did.